This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
With recent advances in precision manufacturing, there has been an increasing demand for the development of efficient and accurate micro-level 3D metrology approaches. A structured light (SL) system with digital fringe projection technology is regarded as a potential solution to micro-scale 3D profilometry based on its capability of high-speed, high-resolution measurement. To migrate this 3D imaging technology into micro-scale level, a variety of approaches were carried out, either by modifying one channel of a stereo microscope with different projection technologies, or using small field-of-view (FOV), non-telecentric lenses with long working distance (LWD).
Apart from the technologies mentioned above, an alternative approach for microscopic 3D imaging is to use telecentric lenses because of their unique properties of orthographic projection, low distortion and invariant magnification over a specific distance range. However, the calibration of such optical systems is not straightforward especially for Z direction, since the telecentricity will result in insensitivity of depth changing along optical axis. Tian et al. in U.S. Pub. App. 20160261851 (incorporated herein by reference in its entirety into the present specification) described a system that uses a translation stage for Z-axis calibration. The orthographic projection model in Tian et al. is uses an intrinsic matrix (a transformation matrix from camera lens coordinate system to camera image (CCD) coordinates system) and extrinsic matrix (a transformation matrix from world coordinate system to camera lens coordinate system). Within this method, dimensions along X and Y are calibrated through the relationship between pixel coordinate and physical coordinate of feature points on a calibration target (e.g. center points of a circle array); Z value is determined by the linear relationship between phase and depth value (in the form of positional information of a translation stage). This method requires a high precision translation stage that is expensive. This technology has shown the possibility to calibrate a telecentric SL system analogously to a regular pin-hole SL system.
There is, therefore an unmet need for a novel approach to a calibration method using a calibration target in a telecentric three dimensional imaging system without a translation stage which does not require positional knowledge of the calibration target.